Choosing the Right Dispersant in Paints

The Importance of Dispersant Selection in Paints Color is a critical concept in the paints industry. The pigments used in color formulation have among the highest raw material costs in many paint systems. Working to maximize yield from the roles pigments play within the system, dispersion of pigments and ensuring these dispersions remain stable are among the most studied areas. The uniform appearance of the paint surface is highly important from a consumer perspective. Paint defects documented in the literature, such as floating and flooding problems, occur in colors made with multiple pigments. Floating creates color differences due to the separation of different pigments or their failure to distribute homogeneously (vertical segregation). Flooding creates color differences due to one or several pigments rising to the film surface (horizontal segregation). Figure 1 shows a schematic representation of floating and flooding defects. When dispersants unsuitable for or incompatible with the system are used, floating and flooding problems on the paint surface lead to color differences in the film surface as well. At the same time, when color pastes containing different dispersants are used together, flooding and floating problems caused by incompatibility occur. For this reason, using the same dispersant across different pigments where possible will help reduce these problems. This is illustrated in Figure 2. The most critical step in preventing these conditions arising from the combination of different pigments is the stage at which pigment dispersions are created. In the dispersion stage, dispersant is added to the paint in small amounts alongside the binder, pigment and solvent, helping improve paint properties (viscosity, storage stability, etc.) during production and storage processes. Dispersants achieve this improvement by keeping pigments permanently separated from one another without flocculation. However, the correct selection of other raw materials in the paint structure will also affect the dispersant's performance. Thus, through selection of the most suitable dispersant for the system, flooding and floating problems can be minimized. Dispersants prevent pigment particles from aggregating through mechanisms such as electrostatic, steric and electrosteric (Figure 3). The first requirement in ensuring stabilization is that dispersants remain on the pigment surface, but this is not easily achieved. Electrostatic stabilization is an effective mechanism particularly applicable in polar systems (water-based) and utilizes the repulsive force of different charges. Steric stabilization applies in both solvent-based and water-based systems; particles cannot approach each other or form bonds due to polymeric chain structures. Electrosteric stabilization comprises a combination of both mechanisms. Multiple different test methods exist to determine the appropriate dispersant for the paint system to be prepared. Initially, the compatibility of dispersants with the system should be tested. For a good dispersant to perform its function, it must have good compatibility with the system. Compatibility can be divided into wet and dry. For this, 5% dispersant is added by weight to a resin/solvent mixture or a solution is prepared at a 9:1 ratio of resin solution to dispersant, and the resulting mixture is examined visually. Dispersants compatible with the system yield a clear/transparent appearance, while incompatible dispersants produce cloudiness. This section can be defined as wet compatibility. The transparent system is placed in a 60°C oven and the solvent in the structure is allowed to evaporate. If a hazy appearance is obtained after this stage, dry compatibility is inadequate. Obtaining a transparent appearance indicates a mutually compatible system. Another test method is accelerated stability testing. For this, paint containing the dispersant being studied is kept in 50-60°C ovens for 10-14 days, then examined visually and rheologically. Additionally, three other commonly used methods exist. These tests are rub-out, pour-out and pigment shock tests.

Rub-Out Test

In this test method, paint mixed with a white base is drawn onto white application paper. When the paint reaches a tacky state just before drying, circular rubbing is applied to the surface. The color difference between the portion where rubbing was not performed and where it was performed is observed. If color darkens after rubbing, it is concluded that the pigment or pigment paste is unstable. In an unstable system, even small mechanical force such as rubbing causes flocculated particles to separate from one another and reduce to smaller sizes, leading to the formation of different colors. Figure 4 shows the difference between stable and unstable systems to which the rub-out method has been applied.

Pour-Out Test

In the pour-out test, the paint system being studied is adjusted to application viscosity and applied to a panel. The remaining paint is poured onto the other half of the panel at a 45° angle, left to dry according to system expectations, and the two different sections are then compared. No shear force is applied in this test. From a theoretical standpoint, considering that pigment movement is faster in paint adjusted to application viscosity, flocculation can be detected more easily. Paints lacking adequate stability may exhibit low color strength, color stability problems, flooding and floating issues.

Pigment Shock Test

In this test method, paint viscosity is adjusted and the pour-out test is performed, with the resulting outcome examined visually. The paint manufacturing process is complete, and while the product is in the viscosity adjustment stage, the test is performed by slowly adding solvent/water under high agitation. If not added slowly, it causes the dissolution of dispersant molecules surrounding the pigments, and once pigment particles are freed, they bond with the nearest particles. Pigment clustering results in flocculation in the paint. Even if the same condition is observed despite this process occurring slowly, it is determined that the pigment has not been well dispersed. Specialized teams have been established within the color unit of the Kanat Boya Research and Development Center. One of this unit's activities is the selection of dispersants which are highly important in water-based and solvent-based systems, and the examination of the performance of these dispersants in paint systems. References 1. Bieleman, J. (Ed.). (2008). Additives for coatings. John Wiley & Sons. 2. Koleske, J. V. (2012). Paint and coating testing manual 15th Edition of the Gardner-Sward Handbook. ASTM International: West Conshohocken, PA, USA. 3. Müller, B., & Poth, U. (2017). Coatings formulation. In Coatings Formulation. Vincentz Network. 4. Pirrung, F. O., Quednau, P. H., & Auschra, C. (2002). Wetting and dispersing agents. CHIMIA International Journal for Chemistry, 56(5), 170-176. 5. Streitberger, H. J., & Goldschmidt, A. (2018). BASF handbook basics of coating technology. European Coatings. 6. Tracton, A. A. (2005). Coatings technology handbook. CRC press. Cansu Çekin Research and Development Chemist Kanat Boya